The production of liquid crystal displays such as, for example, active matrix liquid crystal display devices (AMLCDs) is very complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled. The downdraw sheet drawing processes and, in particular, the fusion process described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both to Dockerty, are capable of producing glass sheets that can be used as substrates without requiring costly post-forming finishing operations such as lapping and polishing. Unfortunately, the fusion process places rather severe restrictions on the glass properties, which require relatively high liquidus viscosities.
In the liquid crystal display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred because of their ability to transport electrons more effectively. Poly-crystalline based silicon transistors (p-Si) are characterized as having a higher mobility than those based on amorphous-silicon based transistors (a-Si). This allows the manufacture of smaller and faster transistors. P-Si displays are at the core of state-of-the-art handheld devices. The polysilicon thin film transistor array consumes very low power, permits very fine features (critical for small displays), and provides high brightness.
The process used to make polysilicon TFTs invariably includes a thermal excursion to quite high temperature to encourage the silicon to crystallize. In some processes, temperature alone is used to produce crystallization, and in such processes the peak temperatures are very high, very typically greater than 650° C. compared to the 350° C. peak temperatures employed in the manufacture of a-Si transistors. At these temperatures, most AMLCD glass substrates undergo a process known as compaction and will deform excessively unless supported from below. Compaction, also referred to thermal stability or dimensional change, is an irreversible dimensional change (shrinkage or expansion) in the glass substrate due to changes in the glass' fictive temperature. The magnitude of compaction depends both on the process by which a glass is made and the viscoelastic properties of the glass. In the float process for producing sheet products from glass, the glass sheet is cooled relatively slowly from the melt and, thus, “freezes in” a comparatively low temperature structure into the glass. The fusion process, by contrast, results in very rapid quenching of the glass sheet from the melt, and freezes in a comparatively high temperature structure. As a result, a glass produced by the float process possesses less compaction when compared to glass produced by the fusion process. In the glass product itself, the compaction ultimately may produce poor registry with the color filter and, if large enough, adversely affect device performance. Thus, it would be desirable to minimize the level of compaction in a glass substrate that is produced by a downdraw process. A commercial glass product, Jade™ (Corning Incorporated, Corning N.Y.), was developed expressly to address this problem. It has a very high annealing point compared to conventional amorphous silicon substrate glasses, and thus shows low compaction even when reheated above the strain point of conventional amorphous silicon substrates.
Laser recrystallization was developed to avoid the very high temperature step of conventional p-Si processes. In this process, the substrate is still heated to elevated temperature, but a laser is used to assist in crystallization via very localized heating. This allows for processing at lower absolute temperature for shorter periods of time, thus reducing cost and increasing throughput. Laser recrystallization can be used to produce state-of-the-art p-Si displays with low power consumption, high resolution and high brightness. There are also lower-temperature p-Si processes that can be used for less demanding applications. In these, either the hold time at peak temperature is much shorter than for a high-resolution p-Si display, or the peak temperature is much reduced with a longer hold time, or with nucleating agents added to the silicon to enhance crystallization and growth. Even with this lower peak process temperature and time, a-Si substrates made via the normal fusion process show excessive compaction. While draw speed can be reduced to lower the fictive temperature, and thus enhance compaction performance, this significantly reduces through-put and thus substantially increases the cost of producing the would-be p-Si substrate.
The very high anneal point of Jade™ is much higher than would be required to produce acceptable compaction for these applications. Furthermore, the visco-elastic properties of Jade™ result in high production costs, and thus it may become prohibitively expensive to use in these applications. Even if process adjustments could be used to drive manufacturing costs down, Jade™ has quite high density compared to conventional amorphous silicon substrates (2.63 g/cc vs. 2.38-2.55 g/cc), and very different acid durability than conventional a-Si substrates. As a result, an AMLCD panel manufacturer who wants to chemically thin Jade™ to compensate for its high density cannot do so in a conventional chemical thinning process designed for a-Si substrates. This adds to the panel manufacturing costs.
What is desired is a fusion-compatible glass with an annealing point between an a-Si substrate (˜720° C.) and Jade™ (˜785° C.) with a density and acid durability comparable to mainstream a-Si substrate products.